Keys
Some doors in maps require keys. You’ll get the "Breach" option when you approach them without the key. If you have the key anywhere in your inventory (including your secure container), you'll get the "Unlock" option. Keys may be reused as many times as you like. Some doors will only give you the "Breach" option but don't require keys.
One example is the door in Factory on the 3rd floor next to the office (which is conveniently named Breach Door). Right next to that door is "Locked Room", which requires the most valuable key in the game (the coveted “Factory Key”). My reference guide is a list of keys and their respective uses (Key Guide).
Another key guide, created using the information provided by the UWS community and compiled by The Velvet Gentleman, is quite helpful (UWS Key Guide).
Repairs
Items in the game each have a durability that can be located by double-clicking the item in your inventory, stash, or hands. So far, the only items you're likely to repair are armor items (discussed below). You’d only improve something other than armor because the traders won't buy any severely damaged items.
Two people can repair your items, Prapor and Skier. Skier costs a lot more, but he reduces the durability by more minor, which is very important for armor. Each time you repair your item, the max condition decreases, and so as you improve the same item again and again after a lot of use, you'll eventually have something that's nowhere near as good as a brand new item would be. One example of this is using the same PACA armor many times.
Repairing it every time it takes a hit will eventually decrease its effectiveness from 30/30 to 24.5/24.5. The difference may not seem like much, but it can be crucial.
Insurance
One fun feature of EFT is Insurance. You can pay a certain amount of EFT roubles to insure your items, meaning if they're lost on a map, whether by you dying or you just leaving them on the ground somewhere, you have a chance of getting them back. To get insured items back, they must not have been extracted off the map (meaning you won't get your items back if another player or player scav takes them and manages to extract with them).
Insurance can be pretty costly, especially if you insure through a Therapist. There are some ways of limiting the cost, though. One way is to only insure through Prapor. Prapor requires you to be insuring at least 1,000 RUB worth of items to use him. This is easiest to do by insuring several items simultaneously through the insurance screen before you enter the game lobby.
Prapor also offers a significant insurance discount the higher your trader level is with him. By the time you've reached trader level 4 with Prapor, he ensures items for 1/6th of the cost of using the Therapist. If you don't get the amount required with Prapor, you can always back out to your character screen, grab a couple of other items to insure, go back to the insurance screen, and return those items to your inventory.
Armor
As in the time of this creation, the armor procedure in Tarkov is staying reworked. This section will almost certainly be out of date by the time you read this. However, I'll leave it in for now. There are three armor items in Tarkov right now. Armor usefulness is based on the durability of the armor piece combined with the armor class.
When using armor, every hit you take will reduce the condition of your armor. When its condition reaches 0, it becomes useless. You can check the current situation of your armor on your inventory screen. Depending on the armor piece, you'll see 30/30 or 64.8/70.
The number on the left is your armor's current condition, while the number on the right is its max possible condition. The max possible infection of your armor reduces with every time it’s repaired (section on repairs above). Though this isn't confirmed anywhere, to the best of my knowledge, armor class determines how much damage the armor takes each hit.
There are a few ways to give yourself a slight advantage when facing someone wearing armor. The first way is simple preparation. Each ammunition type in the game has a different chance to penetrate armor and do additional damage. Check the reference sheet Ammo Guide to find what you prefer.
Bring ammunition that works for both armored and unarmored targets. If someone is wearing PACA or Fort Armor, aiming for their chest isn't going to get you very far. If they're not wearing a helmet, aiming for the head is good, but aiming for the legs is better. There aren't any armor pieces to cover the legs, and 5-10 shots to their legs will at the least cripple their movement speed, but it also has an excellent chance to kill them outright!
PACA
The most common is PACA armor. PACA is a soft armor designed to help protect against pistol rounds. It can take a few rifle rounds as well but is much less effective against them. PACA will make a big difference in survivability, but you can still die quickly. PACA covers your stomach and chest, but not your arms, legs, or head.
The PACA has a max condition of 30/30. PACA is going to become your best friend. It's cheap enough to take it in anytime you're bringing a decent weapon. PACA will significantly increase your chance of surviving a firefight and doesn't reduce your character's running speed as much as Fort Armor.
Kiver
The next item is the Kiver Helmet. This basic helmet can save your life. They're relatively rare at the moment. The only sure way to get them is trading in 5 Air Filters to Skier. Like PACA, they don't do as well against rifle rounds as pistols, but they can take more punishment. The Kiver Helmet only protects the Head area (I'm unsure about the neck). The Kiver has a max condition of 40/40.
Fort Armor
The last item and one of the rarest items in the game in its current state is the Defender Armor. Also known as Fort Armor, Defender is the most significant and meanest armor. It can take a full magazine of pistol ammo and still be in good condition. Fort Armor, though, can also take a lot of rifle rounds. Fort Armor protects your chest, stomach, and arms. Fort Armor isn't currently found on AI scavs because the developers are reworking it. The only way to acquire it is by trading 7 Gold Neck Chains. Fort Armor has a max condition of 70/70.
Desync
Not a part of game mechanics, yet vital information nonetheless, is Desync. Hopefully, by the time you read this guide, Desync will be a thing of the past. If not, then here is what you need to know. Sometimes you may shoot someone in the brain 30 times, and they still won't die. Sometimes instead of dying, they'll kill you. Sometimes, you may just randomly die with no one anywhere near you. This is Desync, don't let it ruin your day.
Often what looks like hacking is just Desync. Other times, Desync means you have to restart your game. The only way to learn how to play around with Desync is through experience. The advice I CAN give you is this: If you can’t extract at an extraction spot, wait a few minutes. If, after a few minutes, you still can’t remove it, try moving to a different extraction (if that’s an option). If that is NOT an option, try alt-f4ing and reopening the game.
You WILL die to Desync. You WILL lose stuff to Desync. It happens to us all. One method of detecting Desync is trying to move an item to an open slot in your inventory and counting the number of times the icon flashes before it moves to the new place. Each moment is about 1 second of Desync.
(Note on Desync: The developers have recently released a patch that seems to have reduced the frequency of Desync by quite a bit. Now I find I run into Desync in no more than 10% of games.)
Related: Escape from Tarkov Guide: First Runs and Farm Cash!